I agree that testing colder conditions would be useful, but I guess you get into problems with it being ice and snow that you're testing, not the "summer tyres get too hard in the cold" hypothesis.

It's winter if it's noticeably cooler than summer. For instance London's climate has a January average minimum temperature of just under 3C, which is very clearly winter compared to the July minimum of 14C. (figures from Met office site on their Heathrow station.) While the temperature does drop below 0C sometimes, it is not consistently below zero.

In southern England pretty much nobody changes tyres for winter -- you just use the same set all year. Optimising for "2C in the wet" seems about right...